You should write down everything you THINK you know about the phrase or words, and anything that comes into mind in future, as possible keywords. At some point you can get a programmer or find a program to run a script to try all the possible combinations and variants.
Do this. There are people that do this with bitcoin wallets now. It's only a matter of time until they offer these services for Nxt, assuming Nxt itself doesn't die on the vine.